Ink jet printing is a well known technique by which printing is effected without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of droplets of ink to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on the substrate. That technique of non-contact printing is particularly well suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of beverage containers.
As is also well known in the art, the physical characteristics of an ink jet printing ink must be carefully controlled. For example, the ink must flow through the fine jet nozzles without causing clogging. This requires the inks to be of a specific, uniform viscosity. Further, ink jet printing inks must be quick drying and smear resistant and must provide good wettability of the substrate.
Acceptable colored inks have heretofore been formulated to contain, for example, a colorant such as a dye or pigment, a resin binding agent which serves to secure or adhere the colorant to the substrate surface, and a carrier fluid or solvent for the colorant or binding agent. The carrier fluid evaporates upon application of the ink. Some colored inks have also contained ancillary agents such as evaporation retardants, resistivity control agents, and other conventional components.
While acceptable colored inks have existed for use in ink jet printing, a need has continued to exist for an acceptable white jet ink.
A white jet ink must form an opaque layer on the surface of the substrate. Those skilled in the ink jet printing ink art have known that one can use particulates (pigments) to achieve such opaqueness. Titanium dioxide (TiO.sub.2) is a pigment which has been used in such white jet inks, without great success.
An ink jet printing ink composition containing particulates presents special problems For example, the particulates must be ground to have a microscopic diameter, preferably about 4 microns, so that the particulates do not plug the small printer orifices. The particles also must not agglomerate, as agglomeration would cause plugging of the printer orifices and irregular droplet formation. The small size is also necessary to assure that uniform ink droplets are reliably formed in the ink jet printers.
Another undesirable characteristic of pigment particulates is that they settle-out over time. Such settling causes the nozzles to clog, results in unacceptable variation in print density and, hence, causes the ink to have a poor shelf life.
White inks have been made heretofore without pigments, but such non-pigmented inks have not had a satisfactory balance in evaporation properties to achieve the blushing effect needed to enhance the opaque image. Further, the non-pigmented white inks known to date have not been able to maintain a stable, high quality opaque image and excessive variation in the quality of the opaque image has resulted.
U.S. Pat. No. 4,389,503, for example, discloses non-pigmented liquid inks which dry to a reticulated film structure containing microvoids which scatter light incident thereto. Such a composition relies on the fact that after its application to a substrate the solvent/non-solvent ratio of the carrier vehicle shifts so that the deposited ink becomes rich in non-solvent and deficient in solvent for the cellulose ester film forming agent. In effect, what is known as phase-inversion occurs, causing the formation of a film having a plurality of microvoids. One of the problems associated with this ink is that the proper formation of the microvoids is critically dependent upon the conditions under which the ink is deposited. Further, because a delicate balance between the solvent/non-solvent ratio must be retained, the composition must be stored and used only under tightly controlled conditions. This prevents the ink from maintaining the ink solvent balance necessary to maintain a stable image; excessive variation in the quality of the opaque image results.
It is an object of this invention to provide improved non-pigmented jet ink compositions, suitable for use with jet ink printing techniques to provide opaque films.
It is another object of this invention to provide improved non-pigmented jet ink compositions that dry opaque white and which are suitable for use on metal, glass and other non-porous substrates.
Still another object of this invention is to provide a jet ink that dries opaque white which has good stability and shelf life.
It is yet another object of this invention to provide an opaque jet ink composition that is nonsettling and of good opaque print quality.
These and other objects of the invention will be better understood from a reading of the detailed description of the invention, a summary of which follows.